This invention relates to the medical diagnosis of arterial disease, and particularly to detection of vulnerable plaque by means of detection of lipid accumulations within the arterial system.
Atherosclerotic coronary artery disease is the leading cause of death in industrialized countries. An atherosclerotic plaque is a thickened area in the wall of an artery. Typically, patients who have died of coronary disease may exhibit as many as several dozen atherosclerotic plaques; however, in most instances of myocardial infarction, cardiac arrest, or stroke, it is found that only one of these potential obstructions has, in fact, ruptured, fissured, or ulcerated. The rupture fissure, or ulcer causes a large thrombus (blood clot) to form on the inside of the artery, which may completely occlude the flow of blood through the artery, thereby injuring the heart or brain. A major prognostic and diagnostic dilemma for the cardiologist is how to predict which plaque is about to rupture.
Plaque, a thickening in the arterial vessel wall, results from the accumulation of cholesterol, proliferation of smooth muscle cells, secretion of a collagenous extracellular matrix by the cells, and accumulation of macrophages. Eventually, hemorrhage (bleeding), thrombosis (clotting), and calcification result. The consensus theory is that atherosclerotic plaque develops as a result of irritation or biochemical damage of the endothelial cells.
The endothelial cells which line the interior of the vessel prevent inappropriate formation of blood clots and inhibit contraction and proliferation of the underlying smooth muscle cells. Damage or dysfunction in endothelial cells is typically produced as a result of injury by cigarette smoke, diabetes, high serum cholesterol (especially oxidized low density lipoprotein), hemodynamic alterations (such as those found at vessel branch points), hypertension, some hormonal factors in the plasma (including Angiotensin II, norepinephrine),), certain viruses (herpes simplex, cytomegalovirus) and/or bacteria (e.g., Chlamydia), and other factors as yet unknown. As a result of these gradual injuries to the endothelial cells, an atherosclerotic plaque may grow slowly over many years. However, it is now well documented that if a plaque ruptures, it often grows abruptly by clot formation, occluding the blood vessel.
When plaque rupture develops, there is hemorrhage into the plaque through the fissure where the surface of the plaque meets the bloodstream. Blood coagulates (forms a thrombus) quickly upon contact with the matrix and lipid of the plaque. This blood clot may then grow to completely occlude the vessel, or it may remain only partially occlusive. In the latter case, the new clot quite commonly becomes incorporated into the wall of the plaque, creating a larger plaque.
Given the enormous impact on public health of acute plaque disruption, much research has attempted to identify those factors which increase the likelihood of a plaque becoming destabilized. The term xe2x80x9cvulnerable plaquexe2x80x9d was coined to denote a lesion at risk of such an abrupt change.
Considerable evidence indicates that plaque rupture triggers 60% to 70% of fatal myocardial infarctions, and that monocyte-macrophages contribute to rupture by releasing metalloproteinases (e.g., collagenases, stromelysin), which can degrade and thereby weaken the overly fibrous cap (Van der Waal, et al., Circulation 89:36-44, 1994; Nikkari, et al., Circulation 92:1393-1398, 1995, Falk, et al., Circulation 92:2033-20335, 1995; Shad, et al., Circulation 244, 1995; Davies, et al., Br Heart J 53:363-373, 1985; Constantinides, J Atheroscler Res 6:1-17, 1966). In another 25% to 30% of fatal infarctions, the plaque does not rupture, but beneath the thrombus the endothelium is replaced by monocytes and inflammatory cells (Van der Waal, et al., Circulation 89:36-44, 1994; and Farb, et al., Circulation 92:1701-1709, 1995). These cells may both respond to and aggravate intimal injury, promoting thrombosis and vasoconstriction (Baju, et al., Circulation 89:503-505, 1994).
Unfortunately, neither plaque rupture nor plaque erosion is predictable by clinical means. Soluble markers, such as P-selectin, von Willebrand factor, Angiotensin-converting enzyme, C-reactive protein, D-dimer (Ikeda, et al., Circulation 92:1693-1696, 1995; Merlini, et al., Circulation 90:61-8, 1994; and Berk, et al., Am J Cardiol 65:168-172, 1990) and activated circulating inflammatory cells are found in patients with unstable angina pectoris, but it is not yet known whether these substances predict infarction or death (Mazzone, et al., Circulation 88:358-363, 1993). It is known, however, that the presence of such substances cannot be used to locate the involved lesion.
Angiograms may be useful for predicting a vulnerable plaque because low-shear regions opposite flow dividers are more likely to develop atherosclerosis (Ku, et al., Arteriosclerosis 5:292-302, 1985). However, most patients who develop acute myocardial infarction or sudden cardiac death have not had prior symptoms, much less an angiogram (Farb, et al., Circulation 92:1701-1709, 1995).
Certain angiographic data has revealed than an irregular plaque profile is a fairly specific, though insensitive, indicator of thrombosis (Kaski, et al., Circulation 92:2058-2065, 1955). Such plaques are likely to progress to complete occlusion, while others are equally likely to progress, but less often reach the point of complete occlusion (Aldeman, et al., J Am Coll Cardiol 22:1141-1154, 1993). Those that do abruptly progress to occlusion actually account for most myocardial infarctions (Ambrose, et al., J Am Coll Cardiol 12:56-62, 1988 and Little, et al., Circulation 78:1157-1166, 1988).
The size of the plaque occlusion is not necessarily determinative. Postmortem studies show that most occlusive thrombi are found over a ruptured or ulcerated plaque that is estimated to have produced a stenosis of less than 50% of the vessel diameter (Shah, et al., Circulation 244, 1995). Such stenoses are not likely to cause angina or result in a positive treadmill test. In fact, most patients who die of myocardial infarction do not have three-vessel disease or severe left ventricular dysfunction (Farb, et al., Circulation 92:1701-1709, 1995).
In the vast majority of plaques causing a stenosis less than or equal to 50% in vessel diameter, the surface outline is uniform, but the deep structure is highly variable and does not correlate directly with either the size of the plaque or the severity of the stenosis (Pasterkamp, et al., Circulation 91:1444-1449, 1995 and Mann and Davies Circulation 94:928-931, 1996).
In view of the dependence of vulnerability on the deep structure of the plaque, studies have been conducted to determine the ability to identify plaques likely to rupture using intracoronary ultrasound. It is known that (1) angiography tends to underestimate the extent of coronary atherosclerosis, (2) high echo-density usually indicates dense fibrous tissue, (3) low echo-density is a feature of hemorrhage, thrombosis, or cholesterol, and (4) shadowing indicates calcification (Yock, et al., Cardio 11-14, 1994 and McPherson, et al., N Engl J Med 316:304-309, 1987). However, recent studies indicate that intra-vascular ultrasound technology currently cannot discriminate between stable and unstable plaque (De Feyter, et al., Circulation 92:1408-1413, 1995).
The relation of the deep structure of the plaque to the rupture process is not completely understood, but it is known that the plaques most likely to rupture are those that have both a thick collagen cap (fibrous scar) and a point of physical weakness in the underlying structure. It is also known that plaques with inflamed surfaces or a high density of activated macrophages and a thin overlying cap are at risk of thrombosis (Van der Waal, et al., Circulation 89:36-44, 1994; Shah, et al., Circulation 244, 1995; Davies, et al., Br Heart J 53:363-373, 1985; Farb, et al., Circulation 92:1701-1709, 1995; and Van Damme, et al., Cardiovasc Pathol 3:9-17, 1994). Such points of physical weakness are thought to be located (as determined by modeling studies and pathologic analysis) at junctures where pools of cholesterol meet a more cellular and fibrous part of the plaque.
These junctures are also characterized by the presence of macrophages (inflammatory cells), which produce heat. Since macrophages and other inflammatory cells release enzymes capable of degrading the collagen and other components of the extracellular matrix, it is thought that they are crucial to the process of plaque rupture or fissuring.
Existing imaging modalities for identifying and treating vulnerable plaque are generally invasive and include coronary angiography, intravascular ultrasound, angioscopy, magnetic resonance imaging, and thermal imaging of plaque using infrared catheters.
For example, temperature sensing elements contained in catheters have been used for locating plaque on the theory that inflammatory processes and cell proliferation are exothermic processes. For example, U.S. Pat. No. 4,986,671 discloses a fiber optic probe with a single sensor formed by an elastomeric lens coated with a light reflective and temperature dependent material over which is coated a layer of material that is absorptive of infrared radiation. Such devices are used to determine characteristics of heat or heat transfer within a blood vessel for measuring such parameters as the pressure, flow and temperature of the blood in a blood vessel. As another example, U.S. Pat. No. 4,752,141 discloses a fiberoptic device for sensing temperature of the arterial wall upon contact. However, determination of temperature by contact requires preknowledge of the site where the catheter is to be placed (i.e., the locus whose temperature is to be determined).
Another type of prior art device is used for visualization of plaque features within a blood vessel, or for estimating the mass of a plaque. For example, U.S. Pat. No. 5,217,456 and U.S. Pat. No. 5,275,594, respectively, disclose the use of light that induces fluorescence in tissues, and of laser energy that stimulates fluorescence in non-calcified tissues. This type of device differentiates healthy tissue from atherosclerotic plaque, but is not reported to be useful for differentiating vulnerable plaque from other, less dangerous, forms of atherosclerotic plaque.
Efforts to develop methods and devices for the study and treatment of vulnerable plaque to date have been hampered by the lack of a reproducible large animal model of vulnerable plaque. Animals do not form plaque spontaneously. In some cases, the use of invasive technologies, such as prior art catheters, is expensive and, because such prior art devices must be threaded through the arterial tree, there is risk of causing damage to the intima that may itself trigger atherosclerotic processes. Accordingly, the need exists for new and better techniques for studying the characteristics of vulnerable plaque, for an animal model of vulnerable plaque, and for relatively inexpensive, non-invasive diagnostic methods for determining the presence of vulnerable plaque in the arteries of an individual.
In accordance with the present invention, there are provided in vivo methods for detection of vulnerable plaque(s) in a subject in need thereof. The invention diagnostic method comprises administering to the subject a diagnostically effective amount of a biologically compatible detectable lipid-avid agent so as to allow the detectable lipid-avid agent to associate with a lipid accumulation in the wall of an artery; and detecting in vivo the presence of the detectable lipid-avid agent attached to the lipid accumulation in the wall of the artery, wherein the detecting is evidence of the presence of a vulnerable plaque.
In another embodiment according to the present invention, there are provided in vivo methods for detection of vulnerable plaque(s) in a subject in need thereof using a macrophage-avid agent. In this embodiment, the invention diagnostic method comprises administering to the subject a diagnostically effective amount of a biologically compatible detectable macrophage-avid agent so as to allow the detectable macrophage-avid agent to associate with macrophages at a lipid accumulation in the wall of an artery; and detecting in vivo the presence of the detectable macrophage-avid agent attached to the macrophages at the lipid accumulation in the wall of the artery, wherein the detecting is evidence of the presence of a vulnerable plaque. Preferably the macrophage-avid agent comprises a lipid-avid agent attached to a macrophage specific antibody, or fragment thereof.
The invention diagnostic methods are useful for determining those individuals with a heightened probability of rupture of a vulnerable plaque with consequent formation of a thrombus in an artery. Thus, the invention methods are used for determining those individuals with a heightened probability of heart attack or stroke, especially of a fatal or near fatal heart attack or stroke. In addition, since pools of lipids in arterial walls are often associated with sites of inflammation and/or infection, including the gathering of macrophages and giant cells, the invention methods are useful for detecting sites of inflammation and/or infection in arterial walls associated with such pools of lipids.
In another embodiment according to the present invention, there are provided method(s) for obtaining an animal model of a vulnerable plaque, said method comprising:
locally depositing a plaque-forming amount of a lipid containing oxidized LDL into one or more blood vessel walls of a live pig while minimizing thrombogenesis so as to form one or more lipid accumulations in the vessel walls, and
allowing the bodily processes of the live pig to react to the lipid accumulations to form one or more structures found in a human vulnerable plaque. One or more arterial structures in the live pig characteristic of a human vulnerable plaque will generally form within about 28 days to about 3 months from the date of lipid deposit
The porcine model of a human vulnerable plaque is a useful animal model for studying the characteristics of human vulnerable plaques in the laboratory for the purpose of developing therapeutic methods of treating such vulnerable plaques.
In another embodiment according to the present invention, there are provided in vitro methods for detection of vulnerable plaque(s) in an arterial section. The invention in vitro diagnostic method comprises administering to the arterial section a diagnostically effective amount of a biologically compatible detectable lipid-avid agent so as to allow at least a portion of the detectable lipid-avid agent to attach to a lipid accumulation in the wall of the arterial section, and detecting the presence of the detectable lipid-avid agent attached to the lipid accumulation in the wall of the arterial section, wherein the detecting is evidence of the presence of a vulnerable plaque in the arterial section. Such methods are useful for studying the characteristics of vulnerable plaques in the laboratory for the purpose of developing therapeutic methods of treating such vulnerable plaques.
Accordingly, it is an object of the present invention to provide effective methods for identifying specific arterial sites at risk for arterial restenosis after angioplasty or atheroectomy.
It is a further object of the present invention to provide effective methods for identifying specific arterial sites associated with lipid pools having, or at risk of forming, sites of inflammation.
It is a further object of the present invention to provide effective methods for detecting transplant vasculopathy.
It is a further object of the present invention to provide methods for identifying vulnerable plaques not characterized by temperature higher than the temperature of surrounding healthy arterial walls, due to areas of extensive scarring, lipid pools where there is no cellular infiltration by macrophages, or areas of hemorrhage and thrombosis which have yet to be colonized by inflammatory cells.
It is a further object of the present invention to provide a reliable animal model of vulnerable plaques that will be useful for study of vulnerable plaques in humans and for development of therapeutic treatments for such plaques.